1. Field of the Invention
The present invention relates to a hydrogen absorbing alloy, and more particularly relates to a hydrogen absorbing alloy suitable for a negative electrode of an alkaline storage battery.
2. Description of the Related Art
Since a hydrogen absorbing alloy which charged and discharged hydrogen was discovered, the application thereof has not been limited to a mere hydrogen storing means, but developed to heat pumps or batteries. Furthermore, in recent years, accompanied with the development of the electronic engineering, the miniaturization and the labor saving for electronic equipment have become important factors. Moreover, recently, a storage batteries using the hydrogen absorbing alloy have come to be used in highlands or cold zones, and batteries having an excellent low temperature property have been required.
In particular, an alkaline storage battery using a hydrogen absorbing alloy as a negative electrode, is put to practical use, and the hydrogen absorbing alloy used in that has also been improved one after another. The LaNi.sub.5 alloy (reference to Japanese Patent Provisional Publication No. 51-13934) has disadvantages that it is easily cracked because of repeated charge and discharge of hydrogen, and therefore, the specific surface area of the alloy increases so that the deteriorated area thereof increases, and further, it is easily corroded by alkaline solution, while it has an advantages that the absorbed amount of hydrogen is large.
Such disadvantage has been improved by displacing part of La of the LaNi.sub.5 alloy with the other rare-earth metal element such as Ce, Pr or Nd, and/or by displacing a part of Ni of the LaNi.sub.5 alloy with metal such as Co, Al or Mn (reference to, for example, Japanese Patent Provisional Publication Nos. 53-48918, 54-64014, 60-250558, 61-233968, and 62-43064).
Then, an alloy for use in which part of La is displaced with Ce or the like, includes a misch metal (Mm) containing approximately 40 to 50 wt % (`wt %` is short for `% by weight`) Ce, and an La-rich misch metal (Lm) containing approximately 50 to 70 wt % La and approximately 10 to 30 wt % Ce.
Generally, in a case where a LaNi.sub.5 type of hydrogen absorbing alloy is used for batteries, it is important that the alloy has a large capacity, and further has a good discharge property at low temperatures, and has a long cycle life for repetition of charge and discharge.
In the above view, by displacing part of La with Ce, the object is tried to be achieved. However, the high rate discharge property at low temperatures or the discharge property is not improved, although the cycle life for repetition of charge and discharge is improved. Furthermore, it is known that displacing Ni with Co, Mn, Al or the like has effect to the efficient discharge property at ordinary temperature and the lifetime.